VARIABLE DENSITY DETECTING APPARATUS

PURPOSE:To prevent malfunction, to obtain a stable output and to make it possible to detect the index of a material to be detected whose variable density ratio is small by receiving both light beams from a light emitting element with two photodetectors, and outputting said output into a detecting circuit. CONSTITUTION:A light emitting element 13 has directivity in two directions toward the following positions: a first illuminated position A where an index 12 of a material to be checked 11 which relatively moves on a main body 10 passes; and a second illuminated position B where a part other than the index 12 passes. First and second reflected light beams 14 and 16 from the positions A and B are received with first and second photodetectors 15 and 17. Outputs VE and VL from the photodetectors 15 and 17 are compared in a detecting circuit 18, and the index 12 is detected.